Significance

Glyphosate is a nonselective herbicide used around the globe for weed control in glyphosate-resistant (GR) and noncrop situations. The extensive and exclusive use of glyphosate has led to the evolution of herbicide resistance in many crop weeds. The molecular target of glyphosate, the 5-enolpyruvlyshikimate-3-phosphate synthase (EPSPS) gene, confers resistance upon amplification and was first documented in GR Amaranthus palmeri. We now report that amplified EPSPS copies in GR A. palmeri are present in the form of extrachromosomal circular DNA molecules (eccDNAs) with various conformations. We discovered that eccDNAs are transmitted to the next generation by tethering to mitotic and meiotic chromosomes. These results represent a report of extrachromosomal structures that drive rapid adaptive evolution in higher organisms.

Abstract

Gene amplification has been observed in many bacteria and eukaryotes as a response to various selective pressures, such as antibiotics, cytotoxic drugs, pesticides, herbicides, and other stressful environmental conditions. An increase in gene copy number is often found as extrachromosomal elements that usually contain autonomously replicating extrachromosomal circular DNA molecules (eccDNAs). Amaranthus palmeri, a crop weed, can develop herbicide resistance to glyphosate [N-(phosphonomethyl) glycine] by amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, the molecular target of glyphosate. However, biological questions regarding the source of the amplified EPSPS, the nature of the amplified DNA structures, and mechanisms responsible for maintaining this gene amplification in cells and their inheritance remain unknown. Here, we report that amplified EPSPS copies in glyphosate-resistant (GR) A. palmeri are present in the form of eccDNAs with various conformations. The eccDNAs are transmitted during cell division in mitosis and meiosis to the soma and germ cells and the progeny by an as yet unknown mechanism of tethering to mitotic and meiotic chromosomes. We propose that eccDNAs are one of the components of McClintock’s postulated innate systems [McClintock B (1978) Stadler Genetics Symposium] that can rapidly produce soma variation, amplify EPSPS genes in the sporophyte that are transmitted to germ cells, and modulate rapid glyphosate resistance through genome plasticity and adaptive evolution.

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